1. Field of the Invention
The present invention relates to a control apparatus for an induction motor, and more particularly to a control apparatus suitable for the constant torque control of an induction motor driven by a variable voltage, variable frequency inverter.
2. Description of the Related Art
A control apparatus for an induction motor is well known, in which direct current (dc) power supplied by a dc power source is inverted by an inverter into alternating current (ac) power of the variable voltage and variable frequency, and the inverted ac power is fed to the induction motor. The inverter as mentioned above is usually called a variable voltage, variable frequency inverter (abbreviated as a VVVF inverter, hereinafter). In such a control apparatus, the VVVF inverter is often operated in such a manner that the ratio (V/f) of the ac output voltage to its frequency is maintained constant. The control of this type is known as a constant V/f ratio control of an inverter.
As one of examples of the application of the induction motor control apparatus as mentioned above, there is a control apparatus for electric rolling stocks having induction motors as driving main motors. In this case, dc power supplied by a trolly wire is inverted by a VVVF inverter into ac power of the variable voltage and variable frequency. The thus obtained ac power is fed to an induction motor. The VVVF inverter is operated in accordance with the constant V/f ratio control, until the output voltage of the inverter becomes its maximum voltage, and thereafter it is operated so as to increase the slip frequency of the motor, until the slip frequency reaches the value limited by the stalling torque of the induction motor.
In such a control apparatus for an electric vehicle, an inverter is controlled in accordance with a command for current to be supplied to an induction motor in order to make the motor produce the constant torque. The reason therefor is as follows.
Torque T.sub.M produced by an induction motor is approximated by the following formula (1); ##EQU1## wherein I.sub.M denotes a motor current, R.sub.2 a secondary resistance of the induction motor, f.sub.s a slip frequency and k.sub.1 a constant.
As apparent from the formula (1), the torque T.sub.M produced by the motor is in proportion to the square of the motor current I.sub.M, since the variation of R.sub.2 caused by the temperature change can be ignored and f.sub.s is automatically determined in accordance with the characteristic of the induction motor on the basis of a required torque. Accordingly, the constant torque control can be achieved by regulating the motor current at a constant value.
In the following, the explanation is made of the prior art control technique, in which the above mentioned torque control is achieved by the so-called pulse width modulation (PWM) control method of a VVVF inverter.
The motor current I.sub.M is at first detected, and the detected I.sub.M is compared with a command thereof to obtain the deviation. A reference for slip frequency of the motor is determined on the basis of the aforesaid deviation. A rotation frequency, which is in proportion to the number of revolutions of the motor, is detected, and the above obtained reference for slip frequency is added to the detected rotation frequency to provide a reference for operating frequency of the inverter, which is equal to the frequency of the output voltage of the inverter, i.e., the voltage applied to the motor.
Further, a modulation factor in the PWM control, which represents a ratio of an amplitude of a voltage control signal to that of a carrier wave signal, is so controlled that the output voltage of the inverter is in proportion to its frequency determined in the manner as described above. Namely, the inverter is controlled so as to maintain the ratio V/f in the output voltage thereof at a predetermined constant value.
In the prior art as mentioned above, however, the required torque could not always be produced. This is caused by the fact that the control of the inverter was carried out on the basis of the current command for the motor current, which is obtained by converting the necessary torque in accordance with the formula (1). There remarkably appears this tendency, especially in the control region where only the frequency is controlled after the output voltage of the inverter reaches its maximum voltage, since the relation between the motor current and the torque is very complicated in this region. Further, because of this complication, it is very difficult and troublesome to prepare a pattern for the motor current command with respect to the speed of a vehicle.